The two most routinely used thermal neutron detectors, the fission detector and proportional Geiger mode detector, are based on technologies that are more than 30 years old. The fission detector is based on the reaction of thermal neutrons with fissionable material (generally isotopes of uranium or plutonium) with the high energy reaction products detected with conventional solid state particle detectors. The proportional detector is based on the reaction of neutrons with either 3He or BF3. Fission detectors have low sensitivity, limited dynamic range and sensitivities are not stable with use.
The majority of neutron detectors in use today are either BF3 or 3He gas proportional tubes. 3He and BF3 detectors have efficiencies similar to the proposed new technique, but require high voltages (1300 V to 2000 V), are susceptible to microphonics and have a dead time of approximately 1 μs limiting their maximum counting rate. The tubes also require an ultra-pure quench gas to achieve sufficient signal-to-noise ratios and suffer from wall effects when particle energy is lost by absorption at the tube walls. In addition, BF3 is a toxic and corrosive gas. While there are many instruments that employ BF3 in the field, manufacturers are moving away from its use.
There are a small number of detectors using a lithium doped scintillator (e.g., lithium glass, lithium iodide, or lithium-loaded plastic). The utility of such devices is limited by gamma ray backgrounds. A need, therefore, exists for an improved method and/or apparatus for neutron detection that has high sensitivity, wide dynamic range, stability and the capability of being calibrated absolutely. The disclosed embodiments have these advantages.
The disclosed embodiments involve phenomena of nuclear physics and of atomic, molecular and optical (AMO) physics, associated with atomic electron excitation as a result of the 3He(n,tp) reaction. This reaction generates a quantity of Lyman alpha radiation that is easily detectable in 3He gas targets. Lyman alpha radiation, at a wavelength of approximately 122 nm in the far-ultraviolet region of the electromagnetic spectrum, is produced by the 2p-1s optical transition in atomic hydrogen isotopes. Such radiation serves as a useful signature for precise neutron dosimetry, leading to single-neutron detection capabilities and compact neutron detectors that can operate over a wide dynamic range without high voltages. Backgrounds from gamma radiation are very low.
The 3He(n,tp) reaction has long been studied in nuclear physics and now is the basis of most thermal neutron detectors used at the National Institute of Standards and Technology (NIST). The small uncertainty in the reaction cross section (0.12%) suggests this reaction as a candidate for the primary standard detector for accurate determination of thermal neutron fluence, but the operational uncertainty of 3He proportional tubes is more than an order of magnitude larger than the uncertainty in the cross section. The disclosed embodiments provide a method for substantially reducing the operational uncertainty of a neutron detector based on the 3He(n,tp) reaction.
The 3He(n,tp) reaction is exothermic at zero incident neutron energy, where it yields (from the nuclear perspective alone) a triton and a proton with combined escape energy of 764 keV. The reaction yields, with unknown branching ratios, hydrogen atoms (H), tritium atoms (T) protons (p), and tritons (t) in a number of final state configurations, including the 2p excited states of hydrogen and tritium, respectively H(2p) and T(2p).
In an ambient environment of 3He gas, Lyman alpha radiation is generated by the following mechanisms: H(2p) and/or T(2p) produced in the initial reaction; higher excited states of H and/or T produced in the initial reaction, followed by radiative or collisional relaxation to 2p states; ground states of H and/or T from the initial reaction followed by subsequent collisions with 3He to produce H(2p) and/or T (2p); and direct production of protons and tritons followed by charge exchange collisions with 3He, leading to both ground and excited states of H and T that then undergo subsequent collisions resulting in H(2p) and/or T(2p). The local 3He environment is transparent to Lyman alpha radiation, which thus allows for its efficient detection by optical techniques. In addition, radiation at the Lyman alpha wavelength is produced by a transition between two excited states of the He+ ion.